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SOLID

Single-responsibility principle
A class should only have a single responsibility, that is, only changes to one part of the software’s specification should be able to affect the specification of the class.

Open–closed principle
“Software entities … should be open for extension, but closed for modification.”

Liskov substitution principle
“Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.” See also design by contract.

Interface segregation principle
“Many client-specific interfaces are better than one general-purpose interface.”

Dependency inversion principle
One should “depend upon abstractions, [not] concretions.”

https://en.wikipedia.org/wiki/SOLID
https://medium.com/@mari_azevedo/s-o-l-i-d-principles-what-are-they-and-why-projects-should-use-them-50b85e4aa8b6

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OOP Concepts

Object − objects have states and behaviors.
Class – defines the grouping of data and code, the “type” of an object
Instance – a specific allocation of a class
Message – sent to objects to make them act
Method – a “function” that an object knows how to perform
Local Variables − variables defined inside methods, constructors or blocks
Instance Variables – variables within a class but outside any method (a specific piece of data belonging to an object)
Class Variables − variables declared within a class, outside any method, with the static keyword
Abstraction – show only “relevant” data and “hide” unnecessary details of object from the user
Encapsulation – keep implementation private and seperate from interface
Polymorphism – process objects differently based on their data type, using same interface
Inheritance – hierarchical organization, share code, customize or extend behaviors